December 9, 2003
Nancy Neal/Don Savage
Headquarters, Washington
(Phone: 202/358-1547)
Bill Steigerwald
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-5017)
RELEASE: 03-397
EXTENSIVE DESTRUCTION POWERS SOLAR EXPLOSIONS
Large-scale destruction of magnetic fields in the sun's atmosphere
likely powers enormous solar explosions, according to a new
observation from NASA's Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) spacecraft.
The explosions, called solar flares, are capable of releasing as much
energy as a billion one-megaton nuclear bombs. The destruction of
magnetic fields, called magnetic reconnection, was a leading theory
to explain how solar flares could suddenly release so much energy,
but there were other possibilities as well. The new picture from
RHESSI confirms large-scale magnetic reconnection as the most likely
scenario.
"Many observations gave hints that magnetic reconnection over large
areas was responsible for solar flares, but the new pictures from
RHESSI are the first that are really convincing," said Linhui Sui of
the Catholic University of America, Washington. "The hunt for the
energy source of flares has been like a story where villagers suspect
a dragon is on the loose because something roars overhead in the
middle of the night, but only something resembling the tail of a
dragon is ever seen. With RHESSI, we've now seen both ends of the
dragon." Linhui is lead author of a paper on this research published
October 20 in Astrophysical Journal Letters.
Magnetic reconnection can happen in the solar atmosphere because it is
hot enough to separate electrons from atoms, producing a gas of
electrically charged particles called plasma. Because plasma is
electrically charged, magnetic fields and plasma tend to flow
together. When magnetic fields and plasma are ejected from the sun,
the ends of the magnetic fields remain attached to the surface. As a
result, the magnetic fields are stretched and forced together until
they break under the stress, like a rubber band pulled too far, and
then reconnect -- snap -- to a new shape with less energy.
The thin region where they reconnect is called the reconnection layer,
and it is where oppositely directed magnetic fields come close enough
to merge. Magnetic reconnection could power a solar flare by heating
the sun's atmosphere to tens of millions of degrees, and accelerating
electrically charged particles that comprise the plasma (electrons
and ions) to almost the speed of light.
At such high temperatures, solar plasma will shine in X-rays, and
RHESSI observed high-energy X-rays, emitted by plasma, heated to tens
of millions of degrees in a flare on April 15, 2002. The hot, X-ray
emitting plasma initially appeared in the RHESSI images as a blob
atop an arch of relatively cooler plasma protruding from the sun's
surface. The blob-and-arch structure is consistent with reconnection,
because the X-ray blob could be heated by reconnection, and the part
of the magnetic field that breaks and snaps back to the solar surface
will assume an arch shape.
These structures have been seen before and hinted at reconnection, but
the observations were not conclusive. However, as RHESSI made images
of the 20-minute long flare, over the course of about four minutes
during the most intense part of the flare, the X-ray emitting blob
exhibited two characteristics consistent with large-scale magnetic
reconnection.
First, the blob split in two, with the top part ultimately rising away
from the solar surface at a speed of about 700,000 miles per hour, or
around 1.1 million km/hr. This is expected if extensive reconnection
is occurring, because as the magnetic fields stretch, the
reconnection layer also stretches, like taffy being pulled. Plasma
heated by reconnection squirts out of the top and bottom of the
reconnection layer, forming the two X-ray blobs in the RHESSI
pictures, when the top and bottom are sufficiently far apart to be
resolved as distinct areas.
Second, in both blobs, the area closest to the apparent reconnection
layer was hottest, and the area farthest away was coolest, according
to temperature measurements by RHESSI. This is also expected if
reconnection is occurring, because, as the magnetic fields break and
reconnect, other magnetic fields nearby move in to the reconnection
region and reconnect as well, since the overall, large-scale field
continues to stretch. Thus, plasma is continuously heated and blasted
out from the reconnection layer. The plasma closest to the
reconnection area is the most recently expelled and therefore the
hottest. Plasma farther away was ejected earlier and had time to
cool.
"This temperature gradient in the hot plasma was the clincher for me,"
said Dr. Gordon Holman, a Co-Investigator on RHESSI and co-author of
the paper, at NASA's Goddard Space Flight Center, Greenbelt, Md. "If
some other process was powering the flare, the hot plasma would not
appear like this." For images, movies, and more information, refer
to:
http://www.gsfc.nasa.gov/topstory/2003/1209rhessi.html
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